1. Field of the Invention
The invention is in general related to an image acquiring method, and more particularly to an area image acquiring method of generating a precise area image of a sheet in an image reading device.
2. Prior Art
Image reading devices, such as a sheet-fed scanner, a combination of a sheet feeder and a flatbed scanner, a multi-function peripheral, a copier and the like, having the sheet-feeding functions, use similar sheet-feeding and image reading methods. As shown in FIG. 1A, for example, an image reading apparatus 10 is composed of a sheet feeder and a flatbed scanner. The sheet feeder includes a feeding roller 11, a paper tray 12 and a switch 13. The switch 13 triggers an optical sensor to send a signal to a control unit of the image reading apparatus 10. When a sheet 14 is placed on the paper tray 12, the sheet 14 actuates the switch 13 and thus the image reading apparatus 10 obtains the information that the sheet 14 is in a position to be transported by the feeding roller 11. The feeding roller 11 rotates and moves the sheet 14 towards the position opposite a scanning module 15.
Because the feeding roller 11 and the scanning module 15 are separated by a distance, as indicated by a dashed line arrow, the scanning module 15 cannot acquire an image until the sheet 14 is moved by the distance. Usually, the path length is known, and it is possible to calculate and control in advance when for the scanning module 15 to start acquiring the image.
However, because the optical sensor is controlled by the mechanical switch 13, a mechanical tolerance Δd1 between the switch 13 and the optical sensor exists. Also, the feeding roller 11 tends to be worn, resulting in an outer diameter error Δd2 which increases with the using time.
When the sheet 14 reaches the position opposite the scanning module 15, the travel error equals to (Δd1□Δd2), which is about 3 to 4 mm in length. In other words, as shown in FIG. 1B, an image 16 contains an obvious leading edge error 17.
The outer diameter error (Δd2) can be reduced by replacing the feeding roller 11, but the mechanical tolerance (Δd1) between the switch 13 and the optical sensor cannot be overcome. In addition, the outer diameter error (Δd2) of the feeding roller becomes greater with the using time.